1. Field of the Invention
The invention relates to a method of correcting a fuel injection amount in a fuel injection apparatus when fuel is injected, and more particularly to a fuel injection amount correction method for a pressure boosting fuel injection apparatus which includes pressure boosting means, and in which a pressure of fuel supplied from a common rail is increased, and the fuel is injected from a fuel injection valve.
2. Description of the Related Art
A so-called common rail fuel injection apparatus is generally known, in which high pressure fuel supplied from a fuel pump is stored in a common rail (an accumulator), the fuel is supplied to an in-cylinder fuel injection valve provided in each cylinder of an internal combustion engine so that the fuel is injected directly into a combustion chamber of each cylinder.
In the common rail fuel injection apparatus, a pressure in the common rail can be controlled to a given value. Therefore, a rate of injection from a fuel injection valve can be controlled to an appropriate value according to an engine operating state so that a good combustion state can be maintained in the cylinder, irrespective of the engine operating state.
Particularly, in a direct injection diesel engine, a fuel injection period during which fuel is injected into the cylinder is limited. Meanwhile, in the common rail fuel injection apparatus, a fuel injection pressure can be maintained at a high value irrespective of an engine rotational speed. Therefore, by using the common rail fuel injection apparatus, it is possible to supply a large amount of fuel into the combustion chamber in a short time so that a high rotational speed and high output can be achieved in the diesel engine.
Meanwhile, since supercharging is generally performed in diesel engines, it is required to further increase the output of the diesel engine by increasing the fuel injection amount and using the supercharging, and accordingly it is required to further intensify the fuel injection pressure.
However, in the common rail fuel injection apparatus, the fuel injection pressure has been already set to a high value (e.g., approximately 180 MPa) that is close to a limit value. Therefore, in order to further increase the fuel pressure, it is necessary to increase a designed pressure of all elements of a fuel injection system such as the fuel pump, the common rail, and delivery pipes. However, it is not practical to increase the designed pressure of all the elements of the fuel injection system, considering problems such as an increase in cost and a decrease in reliability.
Accordingly, in order to solve the problems, a pressure boosting common rail fuel injection apparatus is proposed. In the pressure boosting common rail fuel injection apparatus, a fuel pressure in a common rail is set to a value that is substantially equal to, or lower than a fuel pressure in a conventional apparatus, and a pressure boosting device is employed for further increasing the pressure of fuel supplied to a fuel injection valve from the common rail at a portion that is closest as possible to a nozzle hole of the fuel injection valve.
In the pressure boosting common rail fuel injection apparatus, since the pressure boosting device is used for further increasing the pressure of the fuel supplied to the fuel injection valve from the common rail, the actual fuel injection pressure of the fuel injection valve can be set to a higher value (e.g., approximately 250 MPa) though the designed pressure of the elements such as the fuel pump and the common rail is set to a value that is substantially equal to (or lower than) the designed pressure in the conventional apparatus. Therefore, it is possible to boost the fuel injection pressure while suppressing a large increase in the cost.
Published Japanese Translation of PCT application No. JP-T-2002-539372 discloses an example of such a pressure boosting common rail fuel injection apparatus using such a fuel boosting device.
In the pressure boosting common rail fuel injection apparatus disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372, a pressure boosting unit including a pressure boosting piston is used as a pressure boosting device. The pressure boosting unit is provided between a common rail and a nozzle hole of a fuel injection valve. The pressure boosting piston is formed by connecting a pressure receiving piston with a large diameter, and a pressurizing piston with a small diameter. The pressure boosting piston pressurizes the fuel according to an area ratio between the pressure receiving piston and the pressurizing piston. That is, in the pressure boosting piston, the fuel pressure in the common rail is applied to the pressure receiving piston with the large diameter, whereby the fuel supplied to a pressurizing chamber from the common rail is pressurized by the pressurizing piston with the small diameter. Thus, the pressure of the fuel supplied to the fuel injection valve can be increased to a value higher than the fuel pressure in the common rail, which is decided according to the area ratio between the piston having the large diameter and the piston having the small diameter.
In the pressure boosting common rail fuel injection apparatus disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372, the pressure boosting device is provided in each fuel injection valve, whereby the fuel injection pressure is increased while suppressing a large increase in the cost.
However, a problem may occur when the pressure boosting common rail fuel injection apparatus disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372 is applied to a multi-cylinder internal combustion engine.
In the multi-cylinder internal combustion engine, when torque generated in a cylinder is deviated, the output of the engine may fluctuate, or combustion noise, vibration, or the like may occur. Therefore, it is necessary to minimize the deviation of the amount of fuel supplied to each cylinder, and to make the amount of fuel supplied to each cylinder uniform. Therefore, in the multi-cylinder internal combustion engine, an injection characteristic of the fuel injection valve in each cylinder is adjusted so as to be in a predetermined tolerance, whereby the deviation of the fuel injection amount in each cylinder is reduced.
However, in the pressure boosting common rail fuel injection apparatus, the fuel injection amount is greatly changed depending not only on the injection characteristic of the fuel injection valve, but also on the characteristic of the pressure boosting device. Therefore, when the pressure boosting common rail fuel injection apparatus is employed in the multi-cylinder internal combustion engine, it is necessary to reduce both the deviation of the injection characteristic of the fuel injection valve and the deviation of the characteristic of the pressure boosting device. Thus, adjustment is extremely difficult as compared to when only the deviation of the injection characteristic of the fuel injection valve needs to be reduced.
Also, even if the deviation of the initial characteristic of the fuel injection valve and the deviation of the initial characteristic of the pressure boosting device in each cylinder can be minimized to a specified value, the characteristic of the fuel injection valve and the characteristic of the pressure boosting device vary with use due to wear or deterioration. Therefore, even if the deviation of the initial characteristic is in a specified range, the fuel injection amount in each cylinder may be deviated due to changes in the characteristics caused with use.
In order to prevent this situation, it is necessary to adjust each fuel injection valve and each pressure boosting device so that the deviation of the fuel injection amount in each cylinder is reduced while the fuel injection apparatus is operated. In the pressure boosting fuel injection apparatus, the fuel injection amount in each cylinder is decided depending on both of the characteristic of the fuel injection valve and the characteristic of the pressure boosting device. Therefore, if the fuel injection amount in one cylinder is greatly deviated from the fuel injection amount in another cylinder, it is difficult to determine whether the deviation is caused by the fuel injection valve or by the pressure boosting device, based only on the fuel injection amount. As a result, in the pressure boosting fuel injection apparatus, the deviation of the fuel injection in each cylinder cannot be reduced, and properties of exhaust gas is likely to deteriorate, and vibration or noise is likely to occur when a vehicle is driven.